It is well known that plasma is an electronically exited state of matter in which the electrons from individual atoms are stripped via electrical interactions with the energy source and by collisions with other energized particles. Plasmas are useful at speeding up reactions by transferring the plasma's energy into the vibration translational or rotational energy of reactants. Plasma assisted reactions usually include reforming the reactants. Reactants are reformed by the breaking of bonds and the production of active radicals.
The production of active radicals is the mechanism by which combustion propagates. This process occurs at the expense of energy and time. It therefore stands to reason that eliminating or bypassing the discrete reaction steps could increase the rate of the reaction. This may be achieved through a plasma-assisted reaction.
Moreover, in modern turbofan engines the velocity of the air at the entrance of the combustion chamber is around 150 meters per second depending on the thrust rating of the engine and the atmospheric conditions. The speed of the velocity air entering the combustion chamber is of concern because this 150 m/s flow velocity is much higher than the flame speed of a kerosene air flame, and may make it difficult to sustain combustion. For combustion to occur in a sustainable manner, engine designers generally will reduce both the velocity and pressure of the incoming air before attempting to initiate a combustion reaction with that air. To reduce the velocity and/or pressure of the combustion air, the combustion chamber often will be provided with geometries, which impede the airflow and force it to reduce its velocity. Examples of these geometries are swirl vanes and dilution holes, as well as the shape of the combustion chamber itself. The chamber can be shaped to create an eddy—an area of lower pressure in which combustion is permitted to occur—albeit at the expense of a pressure drop across the combustion chamber. According to basic thermodynamics, this pressure drop in the combustion chamber will reduce efficiency of combustion. As a result of this compromise in even the most efficient jet engines, roughly 10% of the input fuel is left un-burnt.
Moreover, the high temperatures used in the efficient operation of a jet engine also promote the oxidation of atmospheric nitrogen, creating NOx emissions. It has been shown that a way to decrease NOx emissions while simultaneously decreasing fuel burn is to minimize the fuel to air ratio in the combustor. Lean Direct injection (LDI) and Lean Premixed Prevaporized (LPP) combustion systems deliver these air-fuel mixtures. However, the challenge with lean combustion in jet engines is that lean flames are unstable and are subject to blow off, extinction, and thermo-acoustic oscillations, which can cause severe mechanical damage to the engine.